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TCP/IP vs field buses for control applications. The eternal discussion

posted Sep 8, 2008, 11:17 PM by Daniel Berenguer

Why not use TCP/IP for control applications instead of those
complicated field buses?. Indeed, this discussion is appearing every
day in most environments with control needs. But which are the typical
positions around this subject?

Software engineers typically
defend TCP/IP as communication channel for control applications. Their
main argument is that this communication channel can be found in most
computers and appliances nowadays. Moreover, Ethernet and Wi-Fi
hardware interfaces present low prices compared to some years ago.

TCP/IP
is one of the few channels that allow combining big packets of data
with short control-oriented messages. This feature makes this
technology specially suitable for home applications where multimedia
and control can share a common communication system. Besides,
Ethernet-certified cables and RJ45 connectors are very cheap compared
to some control-oriented cabling systems. Most buildings already
contain a LAN infrastructure with the necessary hardware and connecting
IP control systems to those infrastructures is as easy as installing a
new computer in the LAN.

On the other hand, control engineers
don't usually like the idea of relaying on the TCP/IP technology for
controlling critical applications. Furthermore, Ethernet follows a
physical star topology and this complicates the installation process in
networks with big amounts of endpoints. On the other hand, Wi-Fi is
often avoided in industrial environments due to the electromagnetic
noise potentially produced by this technology. Control-specific
technologies usually follow a bus topology, reducing cable runs and
providing a separate communication channel for the control messaging.
Control interfaces as CAN, RS485, Lonworks, LIN and others can be used
with low-power microcontrollers as the communication protocol is often
easier to maintain than the TCP/IP stack. These simple controllers
participate in the bus as listeners, transmitters or both but will
never have to worry about a possible overhead of information coming
from a computer or a media server.

Nevertheless, integrators
know that TCP/IP is an excellent complement to the industrial control
solutions. Ethernet is still the natural way of connecting a PC to a
network and the inclusion of the web technology in remote monitoring
applications force us to mix somehow the best of both worlds.
Multimedia, temperature sensing, web access, binary control, SCADAs,
... when all these applications have to be integrated into a single
solution, then the use of a hybrid network is often the best solution.

The
following schema, extracted from the opnode project, is an example of
integration of TCP/IP and several control-oriented technologies:

As you can see, TCP/IP or "the green network" is used in the above
example to transport multimedia data and also as integration point
between different control technologies. The link between every control
technology and the IP world is relayed on a high performance gateway
that translates and filters the commands coming from both channels,
avoiding then the overhead of data on the control side and reducing the
amount of short commands on the IP LAN.

This architecture is
being widely used in industrial and building applications. No matter
which control bus is installed, it will be connected to the LAN
infrastructure in some point.

After this explanation, a new term
comes into focus: the cost per endpoint. This parameter gives an idea
about the cost (price and power consumption) of controlling a single
endpoint using a given technology or mix of technologies. IP
controllers are more expensive in price and consumption. Thus, the only
way of reducing the cost per endpoint is to add more control points to
the device. This is the reason because most IP controllers are designed
to control important amounts of endpoints whilst other less expensive
control-oriented technologies provide devices controlling just the
temperature of one room.